On categorising induced and natural tectonic earthquakes near new reservoirs

Acquisition of a capability for unambiguous classification of earthquakes near a new reservoir into the twin categories of reservoir-induced and natural tectonic earthquakes is still of considerable practical importance from the seismic hazards point of view. We show how this classification may be carried out in the general 3D case by focussing attention on the perturbation of frictional stability of the causative fault of an earthquake under the influence of the reservoir. We suggest that an earthquake may be put in the induced category only if the reservoir-induced stresses and pore pressure actively promote frictional failure on the fault in the same sense as does the ambient shear stress acting on the fault at the time of occurrence of the earthquake. We consider the case of the main Koyna earthquake of December 10, 1967, to illustrate the application of these ideas. We find that it may have been induced by the Koyna reservoir with pore pressure playing a prominent role in lowering fault stability.